1. Field of the Invention
The present invention relates to a fuel injection system and fuel injecting method for an internal combustion engine, and more particularly to a fuel injection system in which injection valves have been provided on the upstream side and on the downstream side respectively with a throttle valve interposed therebetween.
2. Description of Background Art
When the fuel injection valve is provided upstream from the throttle valve, the volumetric efficiency is improved because heat is taken from intake air when injection fuel vaporizes. Therefore, the engine output can be increased as compared with when the fuel injection valve is provided downstream from the throttle valve. On the other hand, when the fuel injection valve is provided on the upstream side, a distance between its fuel injection port and the combustion chamber becomes inevitably longer. As a result, when fuel injection is provided on the upstream side a response lag in fuel transport occurs as compared with when the fuel injection valve has been provided downstream from the throttle valve.
There has been disclosed in, for example, Japanese Patent Laid-Open Nos. 4-183949 and 10-196440, a fuel injection system in which fuel injection valves have been provided upstream from and downstream from the intake pipe respectively with a throttle valve interposed therebetween, in order to improve the engine output and cope with the response lag.
FIG. 10 is a cross-sectional view showing a major portion of a conventional internal combustion engine in which two fuel injection valves have been arranged, and with a throttle valve 52 of an intake pipe 51 interposed, there are arranged a downstream fuel injection valve 50a on the side portion of the downstream side (engine side) and an upstream fuel injection valve 50b on the upstream side (air cleaner side). A lower end portion of the intake pipe 51 is connected to an intake passage 52, and an intake port 53 facing a combustion chamber of this intake passage 52 is opened and closed by an intake valve 54.
An injection quantity from each fuel injection valve is determined on the basis of a plurality of parameters including a throttle opening and an engine speed. In a state in which the throttle opening is small, the injection quantity is restricted. According to the above-described conventional technique, however, an injection port of the upstream fuel injection valve 50b points to the throttle valve, and in the upstream fuel injection valve 50b, a response lag occurs because the distance between its fuel injection port and the combustion chamber becomes far.
Therefore, when the throttle valve 52 is abruptly closed to a totally-enclosed state or is closed with a large rate of change in an injection-valve closing direction although not up to the totally-enclosed state, fuel injected from the upstream fuel injection valve 50b adheres to the throttle valve 52 and remains.
Therefore, when the throttle valve 52 is opened next, at that time, not only fuel injected from each fuel injection valve in response to the throttle opening, but also the fuel which has remained at the throttle valve 52 is fed into the combustion chamber at the same time. Therefore, there was a possibility that the fuel quantity becomes excessive to the intake air quantity.